Currently, the season for applying epoxy maintenance paints in the northern sections of the United States is restricted to the late spring, summer and early fall since such epoxy/polyamide paint systems do not cure satisfactorily at the low temperatures encountered in those areas during the rest of the year. Additionally, areas such as Alaska, the North Sea region and other more polar areas usually experience such climate conditions that make it virtually impossible for such epoxy/polyamide systems to be used satisfactorily for much of the year.
Accordingly, there is a compelling interest in finding a hardener system which can cure epoxy resin compositions satisfactorily at temperatures of 25.degree.-50.degree. F. (-4.degree. to 10.degree. C.) with curing times of within 24 hours.
The epoxy resin compositions containing such a hardener system must have a reasonable working life of some 6-8 hours as well as providing good cured properties particularly corrosion and chemical resistance. The gloss and color retention of the cured coatings must be at least equal to and preferably better than coatings prepared with a conventional epoxy/polyamide composition. The chemical resistance of the coatings prepared from the low temperature cure system must be at least equal to those obtained from the conventional epoxy/polyamide coatings.
Standard epoxy resin systems do not cure satisfactorily at low (40.degree. F., 4.degree. C.) and consequently limitations are imposed on the use of such systems in some applications. Relatively little mention is made in the prior art of curing epoxy resins at even moderately low temperatures.
U.S. Pat. Nos. 4,490,510 and 4,574,145 describe a new class of epoxy curing agents which are the reaction product of urea, formaldehyde and a polyamine. Epoxy resin compositions containing said products as hardeners can be cured at moderately low temperatures from 15.degree. C. (59.degree. F.) up.
U.S. Pat. No. 2,864,775 pertains to the adducts of aliphatic polyamines and aliphatic monoepoxides. These adducts are useful as hardeners for the curing of epoxy resin compositions at temperatures from 20.degree. C. (68.degree. F.) up. The speed of cure is slow at the low temperature and curing at temperatures of 40.degree.-200.degree. C. are recommended for best results.
Some commercially available materials designed to be reactive curing agents for the low temperature curing of epoxy resin compositions are available from the Cardolite Corporation of Newark, New Jersey. These materials are the reaction products of phenol, formaldehyde and an aliphatic amine. These are Mannich bases or the so-called phenalkamines. These hardeners are touted as curing agents for curing epoxy resin compositions at temperatures as low as 45.degree. F. (7.degree. C.).
U.S. Pat. No. 3,538,184 relates to adducts of polyepoxides with alkyl-substituted hexamethylenediamine. These adducts are useful as hardeners for epoxy resin compositions. Such epoxy resin compositions can be cured at temperatures as low as 5.degree. C. (41.degree. F.). Curing at that temperature requires some 28-32 hours to complete.
U.S. Pat. Nos. 2,689,263 and 2,864,775 describe the preparation of adducts of amines with monoepoxides to give the corresponding hydroxylalkylamine derivatives.